Recruitment and therapeutic application of macrophages in skeletal muscles after hind limb ischemia

Objective

Peripheral arterial disease can cause not only ischemia but also skeletal muscle damage. It has been known that macrophages (MPs) play an important role in coordinating muscle repair; however, phenotype transition of monocyte-MP in ischemic muscle has not been well defined. Hence, the purpose of this study was to examine the temporal recruitment of MPs and to explore their therapeutic effect on ischemic muscle regeneration.

Methods

Unilateral femoral artery excision was performed on C57BL/6 mice. Myeloid cells were isolated from the ischemic muscles, characterized using flow cytometry. Bone marrow-derived MPs were injected (2 × 10⁶ cells) into the ischemic gastrocnemius muscle 24 hours after injury. Blood flow recovery was measured using laser speckle imaging. Functional outcome was evaluated by assessing the contractile force of ischemic muscles. Histologic analysis included quantification of myofiber size, collagen deposition, number of inflammatory and MyoD-expressing cells, and capillary density.

Results

Neutrophils and inflammatory monocytes-MPs were present at day 1 after injury. The mature MPs then remained elevated as the dominant population from day 5 to day 21 with the observation of regenerating fibers. Functional measurements revealed that the force production was significantly enhanced after treatment with proinflammatory M1 MPs (94.9% vs 77.9%; P < .05), and this was consistent with increased myofiber size, capillary- fiber ratio, and perfusion (78.6% vs 39.9%; P < .05). Moreover, the percentage of MyoD-expressing nuclei was significantly higher at day 4, indicating that M1 MPs may hasten muscle repair. Whereas early delivery of anti-inflammatory M2 MPs improved myofiber size, this was accompanied by persistent fibrosis suggesting ongoing tissue remodeling, and lower force production was observed.

Conclusions

We demonstrated the dynamics of myeloid cells in skeletal muscle after ischemic insult, and the administration of exogenous M1 MPs in a temporally coordinated manner successfully improved angiogenesis and skeletal muscle regeneration. Our results suggested that cell therapy using MPs may be a promising adjunctive therapeutic approach for peripheral arterial disease.